Small cryogenic cooling systems are employed in various demanding applications including military and civilian active and remote sensing, superconducting, and general electronics cooling. Such applications often demand efficient, reliable, and cost-effective cooling systems that can achieve extremely cold temperatures below 80 degrees Kelvin.
Efficient cryogenic cooling systems are particularly important in sensing applications involving high-sensitivity infrared focal plane arrays of electromagnetic energy detectors (FPA's). Generally, an FPA may detect electromagnetic energy radiated or reflected from a scene and convert the detected electromagnetic energy into electrical signals corresponding to an image of the scene. To optimize FPA imaging performance, any FPA detector nonuniformities, such as differences in individual detector offsets, gains, or frequency responses, are corrected. Any spatial or temporal variations in temperature across the FPA may cause prohibitive FPA nonuniformities.
FPA's are often employed in avionics applications, particularly missile targeting and satellite applications, where weight, size, and spatial and temporal uniformity of cryogenic cooling systems are important design considerations. An FPA should operate at stable cryogenic temperatures for maximum performance and sensitivity.
Generally, two types of cryocooling systems exist and have been incorporated into FPA's, recuperative cryocoolers and regenerative cryocoolers. In recuperative cryocoolers, a cooling fluid is cycled in a continuous flow. Typical recuperative flow cycles are Brayton or Joule-Thomson processes. Disadvantageously, the cooling fluid typically requires a heavy and bulky FPA cooling interface and heat exchanger, which is attached to the FPA mounting assembly. Consequently, the FPA assembly requires additional mechanical support to secure the interface, heat exchanger, and cooling fluid. The bulky components and additional support hardware oftentimes requires additional cooling, which increases demands placed on the cooling system. In some instances, the bulky support structure, conventionally thought to improve temperature stability, actually reduces system cooling efficiency. Furthermore, the additional bulky mechanical FPA support hardware may cause alignment problems with an on board optical or infrared system during installation and operation, thereby increasing installation and operating costs.
In contrast to recuperative cryocoolers, regenerative cryocoolers have increasingly been employed. Typical regenerative cryocoolers include the Stirling, Gifford-McMahon and pulse tube types, all of which provide cooling through oscillating pressures and masses flows (e.g., the alternating compression and expansion of a working fluid), with a consequent reduction of its temperature. Conventional Stirling and Gifford-McMahon regenerative cryocoolers use displacers to move a working fluid (usually helium or another ideal gas) through their respective regenerators. The noise and vibration induced by the displacer creates problems, and the wear of the seals on the displacer requires periodic maintenance and replacement. Further, the displacer undesirably contributes to axial heat conduction and shuttles heat loss.
Therefore, it may be desirable for cryocooler devices to generate less vibration and less acoustic noise. It may also be desirable to decrease the number of moving parts used in cryocooler devices and to significantly increase the required maintenance intervals and reliability. Pulse tube cryocoolers are a known alternative to the Stirling and Gifford-McMahon cryocooler types; differing from them by the elimination of the mechanical displacer.
A pulse tube is essentially an adiabatic space wherein the temperature of the working fluid is stratified, such that one end of the tube is warmer than the other. A pulse tube cryocooler typically includes a regenerator comprised of a metallic alloy mesh, balls, granules, or shots and a pulse tube, the regenerator and pulse being connected via a cold heat exchanger. Conventional pulse tube cryocoolers operate by cyclically compressing and expanding a working fluid in conjunction with its movement through heat exchangers. Heat is removed from the system upon the expansion of the working fluid in the gas phase. Pulse tube cryocoolers can be divided into two types based on their drivers. The first type is usually referred to as “Stirling-type”, because this type employs a linear compressor with a piston or a plunger to linearly move the working gas, just as conventional Stirling cryocoolers usually do. In these Stirling-type pulse tube cryocoolers, the frequency of the compressors is identical with the oscillation frequency of the working fluid in the tube. Stirling pulse tube cryocoolers are usually operated at frequencies above 30 Hz.
At temperatures below 10 K, pulse tube cryocoolers typically work with frequencies as low as 1 to 2 Hz. For avionic applications pulse tube cryocoolers typically operate at 35K and have been used as low as 4.5K with cooling usually less than 0.1 W. In order to keep the volume of the compressor small, it is advantageous to decouple the compressor from the pulse tube such that both systems can be optimized independently of each other. The compressor can then be operated at a higher frequency of e.g. 50 Hz to provide a constant high and low pressure region. The compressor then utilizes a valve system that alternately connects the hot side of the regenerator with low and high pressure. The frequency of valve switching can be adjusted to the desired operation frequency of the pulse tube cryocooler and can be chosen to be much smaller than the frequency of the compressor. Since this valve switching is similar to the construction of the above mentioned Gifford-McMahon-refrigerater (G-M-refrigerator), pulse tube cryocoolers with such valve compressors are usually called G-M-pulse tube cryocoolers.
Stirling-type pulse tube cryocoolers, which mainly aim at miniaturization, reliability, long life and high efficiency, are gradually replacing Stirling cryocoolers, especially in military and space fields (such as infrared sensors for missile guidance, satellite based surveillance, atmospheric studies of ozone hole and greenhouse effects). However, it remains desirable to provide improved cryocoolers of this type which decrease the overall cost of manufacture without sacrificing the reliability, life and efficiency of the system.